Amp PT troubles

hywelg

A couple of weeks back the amp packed up late in our jam session. Volume dropped to bugger all, and horribly farty. Took it home and got it on the bench on Sunday. Plugged into the Loop return and found it to be the same, swapped the output valves, no change.

Started checking voltages and they were really low, and one side of the HT was lower than the other by 70v (AC) . Much scratching of head and vaguely remembered something about this thread (good god my memory is crap!) and yes the HT CT  fuse was blown. No obvious reason for it having done that, so possibly it was just a little underspecced, or tired?

I had forgotten to get some more 500mA SloBlo fuses so only had Fact Acting and of course they blew every time. Stuck a 1A SloBlo in there for now and it fires up fine. Got some fuses on order

So, I read Merlin on fusing. Seems its an emprical approach, Fit the smallest fuse that doesn't blow with inrush current, nothing terribly scientific.

The one thing that seems strange is that the CT fuse doesn't cut the HT voltages altogether. Still had 320vDC at node 1. Now Merlin recommends identical fuses in both legs of the HT supply. What will then happen if one of them blows? Is there such as thing as a duplex fuse, where if either side blows the other is tripped to open circuit?

There is space for me to modify the wiring to install two fuses, but I'd rather do a proper job and if there is a better way then I'll do it.

Any suggestions?

ICBM

Rewire it with the HT fuse in the right place, which is not in the PT centre tap…

I would put it after the rectifier and before the first filter cap. There's no need to fuse the HT secondaries individually in my opinion - a short in the rectifier will blow the primary fuse anyway.

martinw

hywelg said:

The one thing that seems strange is that the CT fuse doesn't cut the HT voltages altogether. Still had 320vDC at node 1. Now Merlin recommends identical fuses in both legs of the HT supply. What will then happen if one of them blows? Is there such as thing as a duplex fuse, where if either side blows the other is tripped to open circuit?

I wouldn't take much notice of voltage measurements in those circumstances. I'm not 100% sure, but I think that would be an induced voltage i.e. it's not 'real'.

As regards fuses, do as @ICBM says and fit a single fuse after the rectifier. Or leave it as a fused CT; it's not ideal, but it's not the end of the world either. I can't remember coming across any situations where a fused CT has caused any issues. They seem to blow when required.

I always try to use a fast blow fuse on HT, but I've come across many amps with slo-blo fuses fitted, and on my own amps, there have been one or two designs where, on testing, I've found that they absolutely refuse to accept a quick-blow fuse of any sensible value on the HT, and will only accept a slo-blo. No doubt somebody cleverer than me will be along to explain why, but I just accept these things.

hywelg

Thanks chaps. Will rewire it to the DC side of the rectifiers.